This proposal involves detection, quantification and characterization of antigens present in circulating immune complexes in rheumatic diseases. Emphasis is placed on partial purification of immune complexes and detection of antigens contained therein. Sera and synovial fluids will be fractionated by Sephacryl S-300 gel filtration and affinity chromatography with protein A-Sepharose. Appropriate fractions will be subjected to SDS polyacrylamide gel electrophoresis, and the separated antigens will be reacted with antibodies present in patient sera. Control sera of normal individuals will also be used. The interaction between antigen and antibody will be monitored by autoradiography using 125I-protein A. Partially purified immune complexes will also be subjected to affinity chromatography with anti-human IgG-Sepharose. The purified complexes will be radioiodinated and analyzed by SDS polyacrylamide gel electrophoresis and agarose-acrylamide electrophoresis. Attempts will be made to characterize detected antigens by group specific reagents, hydrolytic enzymes and specialized antisera. Antisera raised against newly detected antigens will be used for direct radioimmune assays.